fix: allow null icon16x16 in display schema

[nate-kelley-buster]

Problem

yoto playlist show <cardId> (and anything else parsing card content through GetContentResponseSchema) throws a Zod validation error on any playlist containing chapters/tracks without a custom icon set:

[
  {
    "expected": "string",
    "code": "invalid_type",
    "path": ["card", "content", "chapters", 0, "display", "icon16x16"],
    "message": "Invalid input: expected string, received null"
  },
  ...
]

Root cause

The Yoto API returns display.icon16x16 as null (not omitted) when no custom icon is set. DisplaySchema only declared icon16x16: z.string().optional(), which accepts undefined but not null.

Fix

Add .nullable() to the icon16x16 field.

Verification

Reproduced against a real 25-chapter playlist (all chapters had null icon16x16) — playlist show failed before this change and renders correctly after it.